Regarding Super Troopers, I have to say that the first ten minutes and the last five will have me laughing so hard I can’t even breathe. When the kids are pulled over on the side of the road and they’re just so scared … it’s every stoner’s worst nightmare.
Oh, and the Shenanigans scene. I make that noise every time anyone says “shenanigans.”
Sometimes I like to think I’m a little more high-class than the movie implies, but you can’t help what you find funny! 
Super Troopers absolutely killed me. From the previews, it looked like the dumbest movie ever made, but when I saw it on DVD, I was in hysterics. The syrup-drinking scene cracks me up (and it’s even funnier because that was actually syrup those actors were drinking while shooting the scene).
“I’m gonna pistol-whip the next man who says the word ‘shenanigans’!”
“Hey, Farva, what’s the name of that restaurant you like, with all the goofy sh*t on the walls?”
“Shenanigans?”
“OOOOOOOOOOOOOOOOOOOOOHHHHHHH!” (and much holding-out of pistol butts)
Kills me every time.
Bridget Jones’ Diary - a fairly good movie to me, but I love when Hugh Grant’s character is reciting the dirty limerick in the rowboat
2 weeks notice - very last scene - Hugh Grant in the small apartment
Hmmm…don’t really have a Hugh Grant thing, these two just popped in my mind.
Centerstage - Canned Heat part of the ballet & Higher Ground dance class
Susan
Pulp Fiction
When Vincent’s gun accidentally goes off and shoots Marvin in the head.
Another Super Troopers moment:
“Who wants a mustache ride?”
“I…I do!”
She told me she loved me like a brother. She was from Arkansas, hence the Joy!
In “Dumb and Dumber”, when they’re at the truck stop and Jim Carrey’s character is looking at the writing in the bathroom stall, and it says something like, "For a good time, be here on ____ at ___o’clock, which just so happened to be the date/time he was in there, and suddenly this big trucker fella lets himself into the stall, to the look of sheer horror on Carrey’s face.
I guess the sheer coincedence involved in happening to be in a toilet stall the exact moment some trucker prophecized he would voilate the person occupying the stall is kinda humorous.
In While You Were Sleeping, there’s a scene that has nothing to do with the movie but it’s the part I remember best. It’s the day after Christmas, and the scene is a nice winter morning, with a paperboy on a bike delivering his newspapers. He’s throwing them at the houses as he rides along, when all of a sudden he throws a little too hard and completely wipes out on the sidewalk. It’s just so out of the blue that I was in hysterics the first time I saw it. I laughed for about 5 minutes, then I had to rewind the tape and watch it again. It still sends me into giggle fits any time I see it, even though I know it’s going to happen. Love it.
So far nobody has attempted to answer the OP as to moments that are inexplicably entertaining.
Boondock Saints
Where Connor tears the toilet he’s handcuffed to out of the wall, brings it up to the roof of the apartment building, tosses it off onto one of the Russian mobsters in the alley below, and then jumps off himself.
It’s a great scene, the music is perfect and it adds to the movie a certain subtle super-hero quality.
From Snatch:
Brick Top: In the quaint words of the Virgin Mary… Come again?
Avi: Sit down and shut up, you big bald fuck!
Zebra, Raising Arizona! LOL!!! “I’ll be taking these Huggies and any money you got in the drawer there.”
I know someone’s already mentioned the death of Marvin from Pulp Fiction, but can anyone explain why I laugh so hard every time I see it? He’s dead! He isn’t a bad guy, he doesn’t deserve to get killed, but every time I watch that movie, I’m tittering by “You gotta have an opinion!” Definitely entertaining, apparently inexplicable.
The bit in American Pyscho where he throws the chainsaw down the stairwell. Am I the only one who finds that a teeny-weenie bit funny?
In “Simon Birch” the Christmas play, when the kids are dressed as turtles with wings, and the teacher says they are : Turtle Doves. It cracked me up I couldn’t stop laughing.
There a scene near the beginning of The Thin Man; the case is just starting to unfold and the prime suspect’s lawyer comes to see former detective, and now wealthy socialite, Nick Charles (William Powell) to get him to take the case. The attorney gets a phone call while he’s there, just as Nora (Myrna Loy) comes into the room. Nick reaches over to brush some lint off the front of Nora’s dress, she looks down, and he tweaks her on the nose. And then he does this bizarre little happy dance (which I think is really just him laughing while trying to remain upright; they drink like fish in that movie, alcoholic fish). The phone call reveals a major plot point, but it’s impossible to watch that scene and not have your attention riveted on Nick and Nora.
In Network, the scene where Howard Beale (Peter Finch) exhorts people to go to their windows and yell “I’m as mad as hell, and I’m not gonna take it anymore!” That movie was brilliant when it came out, and it’s even better now. From that point on, everything that happens is right on the very edge of being absurd. But it’s not. It’s at the perfect limit of believable satire. I’ve seen the movie a few times, so I know everything that’s being unleashed by those people yelling out their windows.
Raising Arizona:
“These balloons blow up into funny shapes?”
“No, unless round is funny”
Dude Where’s My Car?:
The entire scene with Brent Spiner and Andy Dick (apparently Andy ad libbed all his lines)
After seeing Dude Where’s My Car in the theater, I went home and watched The Big Lebowski on TV. The Dude gets his car stolen, prompting John Goodman to say, “Dude, where’s your car?”
While in and of itself, not funny, but hilarious in context.
Fear and Loathing: “WAIT!!! We can’t stop here…this is BAT country.”
and after Johnny Depp takes the Adrenachrome, Benicio Del Toro says, “You took too much, man, too much, you took too much.”
Monty Python and The Holy Grail
The “Knights of the Round Table” musical number, where they suddenly cut to a man hung in the dungeon by manacles, who goes ding-dink with his metal cup in perfect time to the music.
I enjoyed the humor of the movie, but for some reason, I laughed so hard at this that I had to leave the room while my friends looked at each other, scratching their heads.
Raising Arizona
While the doctor is showing the young couple a diagram of the female reproductive organs, and helpfully pointing out things with his finger, Nick Cage’s character explains “…her body was a rocky precipice upon which my seed could find no purchase…”
Laughed at that one. Again with the stares.
Monty Python and The Holy Grail
The “Knights of the Round Table” musical number, where they suddenly cut to a man hung in the dungeon by manacles, who goes ding-dink with his metal cup in perfect time to the music.
I enjoyed the humor of the movie, but for some reason, I laughed so hard at this that I had to leave the room while my friends looked at each other, scratching their heads.
Raising Arizona
While the doctor is showing the young couple a diagram of the female reproductive organs, and helpfully pointing out things with his finger, Nick Cage’s character explains “…her body was a rocky precipice upon which my seed could find no purchase…”
Laughed at that one. Again with the stares.
Aww yeah, I love the Sphinx.
“When you can balance a tack-hammer on your head, you will head off your foes with a balanced attack!”
“Okay, but why am I wearing the watermelons on my feet?”
“…I don’t remember telling you to do that.”